Recent events have indicated the need for reliable communication systems during emergencies in underground and hazardous work areas such as coal mines. During a mine disaster the current voice and data communication systems fail or shutdown so the conditions of personnel, environment and equipment is unknown which complicate recovery efforts. Past mining accidents have demonstrated that current communication systems are not sufficient to provide the support required to effectively handle evacuation and rescue operations. The 2006 MINER Act amends the Federal Mine safety and Health Act of 1977 stating underground coal mine operators must provide for post accident communication between underground and surface personnel via a wireless two-way medium within three years. Robust and reliable mine communications are critical for both mining operations and in the event of a mine emergency. The National Institute for Occupational Safety and Health (NIOSH) released a solicitation in late 2006 for an underground communication system that is highly reliable and provides in-mine and mine-to-surface voice and data communications to evaluate wireless mesh network technology as part of an underground communications system.
Prior art mesh communications systems for use in hazardous environments have used the standard Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless protocol to transfer status and multimedia information as well as centrally generated and controlled routing tables to control packet latency in a multi-cast application with a changing network topography. The IEEE802.11 wireless protocol supports an infrastructure mode in which mobile nodes are connected to a coordinating master referred to as the Access Point (AP) and an ad hoc mode in which mobile nodes form peer-to-peer connections with no preconfigured network topology. The original 802.11 protocol had very limited Quality of Service (QoS) mechanisms. A Point Coordination Function (PCF) was specified in which each AP sends a poll to each client one at a time to provide a contention-free transmission opportunity. This media access control (MAC) method has not been implemented in many APs and is bandwidth inefficient since it always requires a poll packet be sent to the client for each high priority packet to be sent to the AP. The 802.11e amendment was incorporated into the 802.11 standard in 2007. It specifies two additional mandatory QoS MAC methods. The first method is the Enhanced Distributed Channel Access (EDCA) method which specifies traffic priority levels and assigned transmission opportunities for each priority level. This method does protect against collisions (and subsequent loss) of high-priority packets due to low-priority packets, however it does not protect against collisions of packets with the same priority from multiple clients. The second QoS method introduced by 802.11e is an infrastructure-only method referred to as Hybrid Coordinator Function Controlled Channel Access (HCCA). This QoS method is similar to the PCF method with the exceptions of the timing of the contention-free transmission opportunities is not limited to certain inter-beacon time slots and the AP is able to poll a client for a particular Traffic Class (TC) to enable session-aware QoS. This HCCA method suffers from the same bandwidth inefficiency as PCF. In addition to the lack of an efficient method to guarantee transmission opportunities for high priority packets between a client and an AP (infrastructure mode) or between two clients (ad hoc mode), the 802.11 solutions do not provide the additional necessary capability to forward said packet to the next destination in the network in an efficient and contention-free manner. Therefore, what is needed is a method and apparatus for establishing dedicated transmission opportunities for each fixed node in a given cluster to forward packets to the next node in the cluster as well as contention-based transmission opportunities for fixed nodes and mobile nodes to join the cluster.
Furthermore, the efficient routing of packets across a mesh network is a vital component in determining the packet latency through the network. Prior art mesh network communication systems for hazardous environments have utilized routing tables generated from a centralized location and periodically propagated across the network using high priority MAC message mechanisms, resulting in a delay in routes being modified as conditions change. There is also a need in the art for an intelligent de-centralized flood routing technique to ensure mobile units can seamlessly roam throughout the network.